Optical record carriers exist in a variety of different formats, with each format generally being designed to be scanned by a radiation beam of a particular wavelength. For example, CDs are available, inter alia, as CD-A (CD-audio), CD-ROM (CD-read only memory) and CD-R (CD-recordable), and are designed to be scanned by means of a radiation beam having a wavelength (λ) of around 785 nm. Red-DVDs, on the other hand, are designed to be scanned by means of a radiation beam having a wavelength of about 650 nm, and Blu-ray Discs are designed to be scanned by means of a radiation beam having a wavelength of about 405 nm. Generally, the shorter the wavelength, the greater the corresponding capacity of the optical disc e.g. a Blu-ray Disc-format disc has a greater storage capacity than a Red DVD-format disc.
It is desirable for an optical scanning device to be compatible with different formats of optical record carriers, e.g. for scanning optical record carriers of different formats responding to radiation beams having different wavelengths whilst preferably using one objective lens system. For instance, when a new optical record carrier with higher storage capacity is introduced, it is desirable for the corresponding new optical scanning device used to read and/or write information to the new optical record carrier to be backward compatible i.e. to be able to scan optical record carriers having existing formats.
Unfortunately, optical discs designed for being read out at a certain wavelength are not always readable at another wavelength. For example, in a CD-R-format disc, special dyes have to be applied in the recording stack in order to obtain a high modulation of the scanning beam at λ=785 nm. At λ=660 nm, the modulation signal from the disc becomes so small (due to the wavelength sensitivity of the dye) that readout at this wavelength is not feasible. Further, there can be a difference in thickness of the transparent cover layers for discs of different formats, which can lead to the undesirable generation of spherical aberration.
A number of solutions have been proposed for the design of an optical scanning device capable of generating predefined wavefronts for the different wavelengths associated with each format.
For instance, Japanese patent application JP-A-2001209966 describes an optical scanning device for scanning optical discs at three different wavelengths. The device comprises an optical axis coupling element for ensuring that the three beams of different wavelength travel along a common optical axis, and a condenser lens for condensing the three different beams onto the information layer of an optical record carrier. The described device has a diffractive part arranged along the common optical axis.
The diffractive part includes a diffraction grating that is rotationally symmetric about the optical axis. The diffractive part has two parallel planes, between which a first layer made of glass and a second layer of birefringent material are provided. The interface between the first and second layers is a pattern of diffractive elements having a single stepped profile, that is rotationally symmetric about the optical axis. The first and second layers are chosen such that the diffractive part forms a first diffracted radiation beam of the zeroth order for a first wavelength, and a diffractive radiation beam of a higher order for the second and third wavelengths. JP-A-2001209966 teaches the use of a solid birefringent material having an ordinary refractive index and an extraordinary refractive index, one of which is the refractive index of the first (glass) layer. The zeroth order beam can be formed by ensuring that the polarisation of the incident wavelength is aligned with the refractive index of the second layer equal to the refractive index of the first layer. In other words, the diffractive part then acts as a transparent parallel plate to the first wavelength.
International Patent Application No. PCT/IB02/05624 in the name of Philips also describes an optical scanning device for scanning three information layers with three respective radiation beams. Each beam has a respective wavelength and polarisation. The three wavelengths differ from each other. At least one of the three polarisations differs from the others. The device comprises a diffractive part including a pattern of elements which have one stepped profile for forming three diffracted beams from three radiation beams, with the part comprising a birefringent material, sensitive to the three polarisations. The stepped profile of the diffractive part is rotationally symmetric about the optical axis. The stepped profile is designed such that the heights of the steps of a pattern element introduce phase changes that equal at least two different multiples of 2π for one of the three wavelengths, and equal at least two different phase changes modulo 2π for at least one of the two other wavelengths.